1. Field
The following description relates to a level shifter. The following description also relates to a level shifter that stably provides a positive voltage (VPP) and a negative voltage (VNN) to a circuit that requires a high voltage for a task like a program operation and an erase operation of a non-volatile memory cell.
2. Description of Related Art
When a voltage of an input signal and a voltage supplied to a circuit are different, a transistor of an input terminal may be always turned ON due to the voltage difference. As a result, a leakage current or a malfunction that interferes with generating output signal normally may occur. A level shifter potentially prevents the malfunction and transmits the input signal by converting a voltage to another, more appropriate voltage level.
A non-volatile memory device uses a voltage that is identical with VDD, or a positive supply voltage, supplied to a semiconductor circuit when reading the memory device. However, when programming and erasing the non-volatile memory device, a positive voltage, generally over 7V, that is greater than VDD, which is generally 3.3V, and the negative voltage, generally less −7V, that is lower than VSS, or a negative supply voltage, which may be a ground voltage, which is generally 0V, is required to remove or inject a charge into a floating gate surrounded with an insulating film. Accordingly, a circuit that applies a voltage to the floating gate is able to output not only VDD and VSS but also the positive voltage VPP and the negative voltage VNN.
A signal controlling program and erase operation uses the source voltage VDD and a ground voltage level, thereby generating different voltage from the positive voltage VPP and the negative voltage VNN that are supplied to a power supply of the non-volatile memory device. Accordingly, the circuit applying voltage to the floating gate uses the level shifter to prevent a malfunction that is generated due to a voltage difference of the input signal.
A level shifter of an alternative technology includes a body or bulk portion of the transistor that is connected to an output terminal, such that the body or bulk portion is connected to the power supply terminal or a ground terminal. When a positive voltage VPP is supplied to the power supply terminal and the negative voltage VNN is supplied to the output terminal, the voltage difference supplied to a drain and body of the transistor becomes the difference between the difference of the positive voltage VPP and the negative voltage VNN. By contrast, when the negative voltage VNN is supplied to the ground terminal and the positive voltage VPP is supplied to the output terminal, a voltage difference supplied to the drain and the body of the transistor becomes a difference between the positive voltage VPP and the negative voltage VNN. Herein, the drain and body form a PN junction, so when a difference between the positive voltage VPP and the negative voltage VNN is larger than the drain-body breakdown voltage, a breakdown phenomenon is generated.
Accordingly, the level shifter according to an alternative technology has an issue of a substantial increase of consumption current and is also unable to normally output a voltage of an output terminal because the drain-body breakdown phenomenon is generated when a voltage difference supplied to the power supply terminal and the ground terminal is larger than the drain-body breakdown voltage.